Monday, December 23, 2013

In the previous part of this series, we did quite a lot. We mocked out any application differences in our ViewModels, we introduced dependency injection and we created base classes for our platform specific pages.

This part will be less complicated, but will again contain a very necessary part if you want to end up with a reusable framework for your Windows Phone and Windows 8 applications. Up until now we have created Views that we can databind to our ViewModels. For this our ViewModels contain either data properties or command properties. It is with this last kind of properties you might have some issues in databinding scenarios. And this doesn't only go for cross platform applications, you will have this issue every time you want to take the MVVM approach.

The thing with command properties is that, out of the box, they are only databindable to, well, Commands. So, a button for instance, poses no problem, since it has a Command property. But what if, for instance, you wish to trigger a command based on a Tap event. This will not be possible by default. But, you can provide some extra pipe-lining to make this possible nonetheless.

What we'll do to get around this, is write our own TriggerAction. A TriggerAction describes an action to perform by a trigger. The good thing is that you can attach a TriggerAction to an EventTrigger. An EventTrigger will then execute your TriggerAction based on a certain event.

To get all this working, first thing you will have to do is add a reference to the Systems.Windows.Interactivity namespace. On Windows Phone you can find this reference under Assembly - Extensions (I had to look for it too). For Windows 8 you will not find this assembly in your references, but you can use this nuget package.

Once you've done this, you will find you can add an EventTrigger for any event you would like. In the following example I have added one for the Tap event on a Border control.

Next step, now that we have our EventTrigger, is write a TriggerAction that translates our event to a command. This is actually not that hard. You notice in the code sample above I use a framework:EventToCommand type, which I provided myself. This is my TriggerAction which has one property, Command. It looks like this.

This class inherits from a TriggerAction for a FrameworkElement (this is just the base class for all possible controls). The first line registers my Command property as a DependencyProperty. This is needed so you can assign values to this property from within your XAML. That is also why, in the actual property, you see the use of GetValue and SetValue - that is just because we are using a DependencyProperty.

The Invoke method in my EventToCommand is more important. It gets executed each time the associated event gets fired. What it does is, it looks wether the Command can be executed and when it does, it executes the Command.

With this, you can more or less databind anything in your views. More or less, because if you want to databind your application bar in a Windows Phone application, you will have to go through some extra effort, which involves a lot more code. You can get a pointer to what needs to be done here.

Next part, I will show you another little trick to easily react to orientation changes.

In the previous part I talked about the MVVM pattern as a great way to share logic in your application. Your ViewModel (and Model) code can be placed in a portable class library, thus making it reusable. But, since it is not possible to put any platform specific code in your PCL, you cannot directly use platform specific services from within a ViewModel (this would make your PCL unsharable between platforms). This poses some challenges, but these can quite easily be overcome. This part in our series will be the most extensive thus far, and will contain the most advanced concepts, like inversion of control, dependency injection, and reflection. But bear with me, it will all be worth it in the end!

Let's first look at one of these platform specific behaviors: Windows Phone and Windows 8 both allow you to navigate from one page to another. The way they do this, however, is different between the two platforms. They both have a Navigate method for this, but the method exists on two different types, and also, the actual parameterlist of this method is different on each platform. You will need to find a solution to get a comparable way of navigating between pages on both platforms. You can do this by wrapping the platform specific functionality with an interface. Your ViewModels can then talk to this interface instead of talking directly to the actual navigation service of each platform.

I have for instance a MainViewModel which acts as start page for my application. From this MainViewModel you can navigate to other pages in the application. For this, the MainViewModel gets a reference to a INavigate interface through its constructor (this is my own INavigate interface, and not the one you can find in the Windows Store or Windows Phone API). Since I will be navigating in my ViewModels and not in my Views, I decided that when I navigate to another page, this is actually the same as navigating to another ViewModel. Also, I want to be able, when I navigate, to send along an additional parameter, which contains extra data for the ViewModel I navigate to.

In the above code you can see it is quite easy to navigate to another ViewModel, and send along extra data (navigate to the SudokuBoardViewModel with a specific game level). But you can also choose not to use this extra data (navigate to the SudokuRulesViewModel).

The interface itself has one extra method for navigating back.

What you will still have to do now is write platform specific implementations of the INavigate service for each platform you want to support. The good thing is, you will need to write this kind of implementations only once. After your first app, you can reuse this effort to a next application (and I will also make sure my own little API will become available on github in the next couple of weeks).

1. Navigating in a Windows Store application

Let's first look at the WindowsStoreNavigator, since this is the easiest to understand. To navigate from one page to the next, you need to make use of your Window.Current.Content property (or rootframe). In a clean Windows Store project you get a reference to this in your OnLaunched event (I set up the entire framework from the OnLaunched handler in the app.xaml.cs file). Next, we will also need some means of associating the ViewModel we wish to navigate to, with a certain View, since navigating in Windows Store applications is done to views and not to viewmodels. For this I will utilize a viewlocator (IKnowWhereYourViewIs). This viewlocator is nothing more than a dictionary that associates views to viewmodels. This way I don't need to code the binding between view and viewmodel directly in the view or in the viewmodel.

As you can see, I ask the viewlocator for the view that is associated with the viewmodel I wish to navigate to. Next I tell the rootframe to actually navigate to this view. The second parameter in the second line of the NavigateTo method instantiates my viewmodel. The inversion of control container has been set up with a list of all my viewmodels. For this I utilized TinyIoc, but you can use any wich IoC you like. The main advantage using and IoC container will be that any dependencies your viewmodel uses, will get injected by the IoC, as long as you registered them at startup.

Navigating with the extra data is a bit more complex. I created a IHandle interface which indicates wether a ViewModel can handle a certain message. If it does, it implements the IHandle interface. Before navigating to the view, I first let the ViewModel handle the actual request.

One more thing missing after navigating to a certain view, is the actual databinding between View and ViewModel. For this I created an extra base class for each View. In the OnNavigatedTo handler, we will bind the NavigationEventArgs, which in our case is our ViewModel, to the Views' DataContext.

And that's it. This allows us to navigate from one ViewModel to another, while sending along data. Also aer we now able to actually databind our ViewModel to the actual View.

2. Navigating in a Windows Phone application

Now that we can navigate in a Windows Store application, let's do the same thing for Windows Phone. As you will see, the API for navigating in Windows Phone is different from the one Windows Store applications use. In Windows Phone you navigate based on a Uri. Also, we will not be able to send along our ViewModel and request as easily. In this case, we will serialize everything before we navigate. Ie. we will serialize our ViewModel type, the actual request and the request type.

Again, we will use a MvvmPage base class, which all our views can inherit from. In its NavigatedTo event handler we will now have to (re)create our ViewModel and request. I pulled out this code to another class which implements ICreateViewModels. Since views cannot use dependency injection, I wrapped the IoC in a singleton, which can give me my viewmodelcreator.

The ViewModelCreator then uses a deserializer and some reflection to recreate the viewmodel and to make it handle the actual request.

The above code is actually the most complex part of the entire framework. An, as said, code like this, you will only have to provide once, after that your can reuse it for your next applications.

Once you got thi plumbing in place, you can navigate between ViewModels in a way that is reusable on Windows Store and Windows Phone applications. It wil make it easier to put your logic code in a shared library and to easily make changes to it that will get reflected on each platform.

Hang on for the next part, where I will talk about some extra helper classes to make MVVM work better for you.

In the previous part I talked about portable class libraries and mentioned the need for a pattern in our code. This pattern will be the Model - View - ViewModel pattern, or MVVM for short. It divides the pieces of your application in more sensible parts and allows you to better define the responsibilities of each individual part. The View for instance will be responsible for nothing more than UI related code. This will be your XAML file. When using MVVM, I try to define my entire user interface in the XAML of my application. I hardly ever put anything in the code behind (except maybe for some UI specific code), all application logic will go someplace else.

The Model in this pattern can be either data you need or services you will be using. These will contain quite some logic and we can write them in such a way that they are unit testable.

The ViewModel will sit between the View and the Model and its responsibility will be to shape the data of the Model in a way that is specific for a certain View. As with the Model, it will also be unit testable.

The ViewModel will contain all the properties for the View to databind to. For these properties you can choose to fire the PropertyChanged event - and thus your ViewModel will implement INotifyPropertyChanged.

The above piece of code shows two properties of one of my viewmodels. The first one is for a collection of items. The second one is for a simple property. Both properties fire the PropertyChanged event. As you can see this is done through a lambda expression. I prefer putting this kind of PropertyChanged event wrapping in a base class for my viewmodels and providing the easier lambda syntax. This provides me with compile time checking (and less runtime errors when I make typos). The way to do this, is shown in the following code example.

Next to simple data properties, your ViewModel can also contain command properties. The View can also databind to these (for instance from the Command property of a Button). A command property will ascertain something gets executed in the ViewModel or in the Model.

As you can see, I use a RelayCommand class, which again makes the creation of command properties easier. The RelayCommand class will implement the ICommand interface and will provide an API by which you can createICommands based on lambdas or delegates. The RelayCommand class looks like this:

When creating commands for your view to databind to, you can also use the CanExecute property, to indicate when a command can be fired, and thus, when the button you databind do should be enabled. This is quite a neat way to automatically enable or disable buttons on your view.

Databinding to all this, is now quite easy from the view.

Using this pattern we get a better separation of concerns in our application. The only thing to watch out for is the danger of your ViewModels getting too big. Keep in mind that the ViewModels' purpose is to shape the data of the Model specifically for a certain View. This means that most of the logic will be the responsibility of the Model, and not necessarily of the ViewModel.

The advantage will be that you can put both Model and ViewModel in your portable class library, giving you the ability to reuse all of the logic between different platforms. And since both Model and ViewModel can be unit tested, you gain a great advantage.

But we will still have to add some extra techniques if we want the ViewModel to be able to use platform specific services. For this we will introduce dependency inversion in the next part of this series.

Tuesday, December 3, 2013

In the previous part of this series, you learned about linked files. While these are a good technique for quickly sharing pieces of code of your application, I prefer using a class library for shared functionality. The problem is however, you cannot use a regular class library in Windows Phone or Windows 8. What you can use though is a portable class library. This is a special kind of class library that makes it possible for you to target multiple platforms.

What you will get when using a portable class library is the greatest common divisor between platforms of functionalities you will be able to use.

For creating one, you can chose 'Portable Class Library' as a project template.

When creating the PCL you will get an additional pop up asking you for the platforms you wish to target with your portable class library.

In our case choosing Windows Phone and Windows 8 will suffice. For Windows Phone you might also choose to target at least version 7.5 or 8. The higher the version you choose the more possibilities you will have in your portable class library. This is due to the fact that Windows Phone 7 doesn't support the entire .Net framework, so you will bump into some coding restrictions when specifying Windows Phone 7. I myself, most of the time, choose at least version 7.5, just for convenience and since I assume most Windows Phone 7 users will have by now upgraded to this free version.

So, now we have a place to put all of our shared code. But, what will this shared code be? It's not like we can put the code behind pages of the XAML files in this class library. So we will have to take the not so conventional path and make use of an additional pattern which will make it possible for us to put the business logic of our application in the shared class library. This pattern will be the MVVM pattern, since it can very easily be used in databinding scenarios and since it gives us a reactive user interface with INotifyPropertyChanged. This will be explained in the next part of this series.

Monday, December 2, 2013

If you want to be able to quickly share things between a Windows Phone and Windows 8 application, you can always use the technique of linked files. Linked files are existing files you add to a project, for instance a file that already exists in your Windows 8 project and which you want to add to your Windows Phone project. But, instead of adding this existing file in the normal way, you add it as link. This is an additional option you can choose in the 'add existing item' dialog box.

This will give you this same file in your other project, but not as a copy. Meaning that if you make a change to the file in one of the two projects, you will see this change reflected in the other project as well.

You can use this technique for reusing code files or for reusing XAML files. But, as stated in the intro of this series, with XAML files you will have to:

remove any platform specific namespaces.

only reuse small pieces of XAML (you will for instance see that for pages the start tag in your XAML is different, for Windows Phone this is a PhoneApplicationPage, for Windows 8 this is a Page tag).

If you use the technique of linked files for sharing code files, you can additionally use partial classes to cope with any platform differences. The shared code you can put in a partial class that you add as a linked file for the other platform. You then add another partial class file (with the same class name of course) in each project with the platform specific stuff. If you don't like partial classes that much, you can also choose to use inheritance for this. You will add your base class as link and create child classes with platform specific code in them.

Another technique for coping with platform differences when using linked files is the use of preprocessor directives. These look like if-statements you put in between your code. They will contain the lines that are specific for a certain platform.

(The above code is courtesy of MVVMLight, a very good starter framework for doing MVVM).

As you can see in the above code snippet, there are some specific if-statements, called preprocessor directives, added. The ones for NETFX_CORE indicate to the compiler the lines of code that need to be compiled for Windows 8. The WINDOWS_PHONE ones will be compiled for Windows Phone. This can help you add platform specific code as well.

With the technique of linked files you can get quite some reuse of code between Windows Phone and Windows 8 applications. You can also use partial classes, inheritance or preprocessor directives to add platform specific code. But I do want to point out the following flaws in using this technique (and this is why I don't prefer using it myself):

Preprocessor directives, especially when used a lot, make it harder for you to read and understand your code (I've had projects going from a couple of preprocessor directives in one file to multiple and thus even more unreadable code).

In the following parts of this series, I will show you another technique for adding platform specific code, that will, at the same time, keep your code cleaner, more readable, more reusable and more maintainable (now, wouldn't that be nice).

Sunday, December 1, 2013

This is Part 0 of a multi-part series blog post. In this series I will give you a walk through of how you can build a cross platform solution that targets both Windows Phone and Windows 8. I will try and give you pointers on how to keep your code as reusable and clean as possible. We will especially look at a couple of patterns and good practices for this. And in the mean time you will get to know some more advanced MVVM.

But before we start, what are actually the problems you face when trying to target two platforms like Windows Phone and Windows 8? They are both XAML based, right, so this shouldn't be too big of a problem. Well, although both platforms are on the path of convergence, meaning moving towards each other, there are still quite some differences you need to take into account.

One of those differences is the actual XAML you can use on each platform. Although the controls on each platform might be the same, they often live in different namespaces. There are also controls that are specific to either Windows Phone - like the PivotControl - or to Windows 8 - like the FlipView control.

This makes sharing your XAML less obvious.

I wouldn't recommend sharing too much of your XAML between Windows Phone and Windows 8 applications Anyway. You will see that you will mostly try and build a UX experience that is specific for the platform at hand. Keep in mind as well that both platforms support quite different form factors. The things that will be most easy to share will be small pieces of XAML - like things you put into user controls or small data templates.

Apart from differences in your XAML you will also see differences in the actual API's you can use from Windows Phone and Windows 8. Both for instance provide you with an API for navigating between pages in your application, but if you look at the actual Navigate call for each platform, you will find that the call takes different parameters.

This makes it less obvious to share code between the two platforms. However, later in this series, I will show you a technique that will give you quite some advantages for reusing for instance your navigation code.

We just have to use a couple of techniques to get around these differences. I will try and give you some pointers in this series. Mainly I want you to get to know some techniques that will allow you to reuse as much of your business logic (the heart of your application) as possible while targetting multiple platforms.

In this series you will find the following parts (and hopefully, if time permits, more to come):

I also recently did a talk on this topic at the 2013 MCT Summit, which got filmed. I hope I can soon add the video of it to this series, so you have some extra reference material there. Other extra reference material to come:

A list of useful links on cross platform development

The slides of my 2013 MCT Summit talk

The start of a little framework you can use to build applications that target Windows Phone and Windows 8 (work in progress for now)

So, hope to see you soon for the first part in this series! I'll keep you posted!